Positron Emission Tomography (PET) is a functional imaging technique that has the potential to quantify the rates of biological processes in vivo. Recently it has become apparent that PET techniques have the potential to play an important role in the diagnosis and management of patients with breast cancer. However, current breast cancer imaging procedures use large, whole-body PED scanners which are far too expensive to be considered for routine screening use. In addition, whole-body PET scanners yield quite poor images of the breast due to the low specific sensitivity and large amounts of scatter and attenuation. The large size and cost per channel of the photomultiplier tubes (PMTs) used in these detectors significantly adds to the system cost and overall size. Recent progress in heavy scintillator and detector developments in have suggested methods of significantly reducing the scanner size, and cost, while improving image quality. In the Phase I, we will address this limitation by fabricating a compact PET detector module which will form the basis of a high image quality PET system for breast cancer imaging. The module will consist of a segmented lutetium oxyorthosilicate (LSO) array end-coupled to ultrahigh gain avalanche photodiode (APD) arrays. The arrays will be manufactured with low-cost modern integrated circuit fabrication procedures while retaining the performance characteristics of RMD s traditional APDs. Standard high gain, APDs are very expensive due to the labor-intensive bevel forming process, making impractical a system PET based on our standard devices. A small-volume PET system based on a large number of monolithic APD/LSO arrays will have a compact design, a significant improvement in sensitivity and image resolution, and cost less than 30 percent of currently available clinical PET scanners. PROPOSED COMMERCIAL APPLICATION A low-cost X-ray compact gamma-ray detector module would find widespread use in such nuclear medicine instruments as planar gamma cameras, SPECT, and PET systems. A low cost, large-area APD would find uses in many low-light level applications as replaments for standard photodectors.